1. Field of the Invention
The present invention relates to a novel cannula and, more particularly, to a cannula which may be suitably adapted for using as a ventricular assist device in heart surgery.
A cannula for heart-lung machine is conventionally used in open-heart surgery. However, the present invention provides a new type of cannula which has been developed for a total artificial heart or a partial or auxiliary artificial heart (to be simply referred to as an artificial heart hereinafter) for use in open-heart surgery based upon totally new technology.
2. Description of the Prior Art
With recent medical developments, the human life expectancy is becoming longer, however, the ratio of cardiac failure to all the various causes of death is also increasing. Artificial hearts are being developed for patients who suffer from serious heart diseases which are incurable by conventional operative techniques or medicine, such as chronic congestive heart failure, acute myocardial infarction, or postoperative low output syndrome. However, artificial hearts are still not commercially available. Moreover, only a few organizations or institutes are studying the subject, and only a little more than one hundred clinical tests have been reported. Meanwhile, the type of artificial heart under development is not limited to one, but various types thereof are being developed. Irrespective of type, any artificial heart, however, will require a novel cannula which in used to operate the artificial heart to assist, during or after the operation, the function of the natural heart of a patient suffering a serious heart problem until the patient's heart starts functioning normally by itself, thereby saving the patient's life.
FIGS. 1A and 1B show conventional tubular cannulae for an artificial heart-lung. Each of these cannulae has a small inner diameter, a straight axis, and openings 10 at an insertion or distal end 12. The cannula is flexible and may be bent two-dimensionally (e.g, into a U-shape). In the cannula shown in FIG. 1A, a metallic coil spring 13 is embedded inside except at a proximal end to be coupled to an artificial heart and at the insertion or distal end 12. In the cannula shown in FIG. 1B, a flange 11 is formed integrally with the cannula portion leading to the openings 10.
With the cannulas shown in FIGS. 1A and 1B, pressure loss is significant due to the small inner diameter, and the shape does not conform exactly to that of the heart, thus failing to allow full functioning of the artificial heart. Furthermore, when the straight cannula is as an inflow cannula, this cannula may press on the lung and prevent the movement of the patient's heart. Therefore, this type of cannula is not suitable as a cannula for an artificial heart. Further the cannula without the flange at the distal end portion as shown in FIG. 1A may become accidentally detached from the heart. In particular, when this type of cannula is used for an artificial heart, it may become more easily detached due to pulsation of the heart over a long time interval, thereby threatening the patient's life.
The first requirement of a cannula for allowing full functioning of an artificial heart lies on its shape. A patient's heart is located in the intrapleural space, i.e., the narrow space between side walls, and beats or pulsates vigorously. Cannulas are roughly classified into two types; an inflow cannula and an outflow cannula. An inflow cannula ejects blood from a left or right atrium, and guides the blood to the artificial heart. An outflow cannula guides blood from the artificial heart to the pulmonary artery or the aorta.
A cannula of either type must be capable of detouring the beating heart in order not to impose any restraint on the movement of the patient's heart, and must be as short as possible in order to reduce the pressure loss to the minimum. In order to satisfy these requirements, it is desired that the cannula has a special shape involving a special curvature or convolution. In addition, the bent portion of the cannula must not be pulsatile due to pulsation of the blood and must not cause the kinking phenomenon.
A cannula must also be able to be applied in various situations. For example, a cannula may be required to be used for males and females, adults and children, and so on. The heart of a patient suffering a heart problem is generally abnormally large, known as hypertrophy of the heart, and the degree of such hypertrophy differs from one individual to another. A cannula which may be used in such various situations has not yet been proposed. However, such a cannula is desired in order to satisfy practical needs.
Pulsation pressure from the human heart may be as high as 200 mmHg and pulsation frequency is 60 to 120 times per minute. A cannula must not allow peristaltic movement due to such pulsation. In a case where a cannula deforms in accordance with the pulsation of the artificial heart, the pulsation of the blood is absorbed by the cannula and the original functioning of the artificial heart is impaired. Accordingly, a cannula must not allow peristaltic deformation in any portion thereof due to blood pulsation during a heart operation.
If a cannula has a large diameter along its entire length, it exerts an excess pressure on the patient's heart to limit its functioning and therefore the artificial heart is reduced in its function of assisting the patient's heart. Thus, a cannula for an artificial heart preferably has a diameter smaller than a predetermined value, at least at that part thereof which directly contacts with the patient's heart. Conversely, the inner diameter of the connecting or proximal end of the cannula which is to be coupled to the artificial heart is preferably kept greater than a predetermined value. If the connecting end does not have a diameter greater than the predetermined value, pressure loss is caused before the blood from the artificial heart reaches the aorta or the pulmonary artery. A cannula must, finally, allow instant adjustment of its length during an operation. This is because the shape of the heart differs from one patient to another as has been mentioned earlier, and the cannula must be adjusted in length for each individual patient.